The present invention relates to a device and method for making dimensional measurements on multi-layer objects such as wafers. It also relates to an imaging device making it possible to locate structures below the surface of such objects, in particular for the purpose of positioning measuring sensors relative to these structures.
The present invention also relates to a method and an imaging device for controllably revealing structures buried in objects such as wafers.
The field of the invention is more particularly but not limitatively that of the measurement and dimensional control of devices in the field of microelectronics, microsystems (MEMs) or integrated optics.
Manufacturing techniques for microelectronics and microsystems (MEMs, MOEMs) are developing towards the production of complex volume structures, which can allow better integration of the functions of these systems in their volume.
These structures are characterized by the superimposition of a number, sometimes a large number, of layers of components, with interconnection tracks (or vias) which connect these layers of components. These techniques belong to what is frequently called “chip level packaging” or “3D integration”.
The layers of components can be produced on separate wafers, which are then superimposed and interconnected.
More precisely, the manufacturing methods can comprise the following steps:                etching of the vias, which are present as holes or trenches opening on only one side of the wafer (the components surface);        metallization of the vias and at least partial production of the conductor tracks and components on the components surface,        thinning of the wafer by polishing (usually by a mechanical method) of the rear surface (i.e. the surface opposite the components surface). The wafer is stuck to a temporary transport wafer in order to obtain sufficient mechanical rigidity. In fact, after polishing, the thickness of the wafer can be reduced to a few tens of micrometres.        
The thinning makes it possible to reduce the thickness of the wafer to a predetermined thickness, or until the vias break through.
It is very important to control the thickness of residual material between the bottom of the vias and the rear surface of the wafer during the thinning operation.
Different techniques are known which make it possible to measure this thickness of residual material.
For example, techniques based on time-domain or spectral-domain low-coherence interferometry are known.
The document U.S. Pat. No. 7,738,113 by Marx et al. is also known, which describes a device making it possible to carry out this measurement with probes based on a scanning confocal technique or chromatic dispersion confocal technique.
However, the problem arises of locating the vias which cannot be seen from the rear surface of the wafer. This problem is not trivial as these vias may be a few micrometres or a few tens of micrometres wide, and it must be possible to accurately position in line with them a measurement beam the diameter of which is not much greater.
It is known to couple point distance-measuring sensors with an imaging system which produces an image of the surface of the wafer, and which makes it possible to accurately position the measurement beams.
These systems do not make it possible to solve this problem of positioning because:                as explained previously, the vias cannot be seen from the rear surface of the wafer;        at the time when the thinning operation is carried out, there are already components and metal tracks which can take up several square millimetres on the components surface of the wafer. These components completely mask the position of the vias, and they are moreover completely opaque, which prevents location of the vias by transparency.        
Beyond this particular problem, the development of the “chip level packaging” techniques results in a need to be able to accurately measure thicknesses or positions of multiple layers of stacked materials.
These layers can be of the order of a micrometre or less up to several hundred micrometres, and there may be a large number of them. In practice, none of the measuring methods previously mentioned (interferometry or confocal) is capable of satisfying all of the specifications for this type of measurement, which in practice leads to having to multiply the measurement devices.
During a wafer thinning operation, it is sometimes also very important to check whether vias have broken through, i.e. whether they are apparent on the polished or thinned surface. This makes it possible in particular to optimize the thinning operation and to stop it when all the vias are “revealed”, i.e. when they appear on the thinned surface.
An object of the present invention is to overcome the drawbacks of the prior art relating to distance and thickness measurements on complex structures.
An object of the present invention is in particular to propose a system which makes it possible to locate vias or similar structures which cannot be seen from the surfaces of a wafer.
An object of the present invention is also to propose a system which makes it possible to carry out residual thickness measurements on vias from the rear surface of a wafer.
Finally, an object of the present invention is to propose a system which makes it possible to carry out thickness measurements in an extensive dynamic range and on a high number of interfaces.
An object of the present invention is also to propose a method and an imaging device making it possible to check, during polishing, whether vias have become apparent.